1. Field of the Invention
The present invention relates to an X-ray generating apparatus and a control method thereof.
2. Description of the Related Art
Among X-ray tubes, there are X-ray tubes that employ a reflecting-type target and those that employ a transmission-type target. In either type of X-ray tube, a target is irradiated with an electron beam that has been accelerated to high speed, and thus an X-ray is generated from the irradiated area. At this time, X-rays are emitted in all directions. Therefore, in many X-ray tubes, in order to block an X-ray travelling in a direction other than that which is necessary, an X-ray shield member of lead or the like is used to cover a chamber into which the X-ray tube has been inserted or an area surrounding the X-ray tube. In Japanese Patent Laid-Open No. 2007-265981, technology is disclosed in which emission of an X-ray in a direction other than that which is necessary, is suppressed by providing a front shield member and a rear shield member.
Here, FIG. 8A shows an example configuration of a conventional X-ray tube 120. An electron beam 201 that has been radiated from an electron source 121 irradiates a transmission-type target 124 via an opening provided in a rear shield member 122. Thus, an X-ray is generated in all directions from the irradiated area.
The transmission-type target 124 is provided with a front shield member 123 on the opposite side as the electron source 121. An X-ray (203) generated from the irradiated area of the transmission-type target 124 is irradiated toward a subject via an opening provided in the front shield member 123. The rear shield member 122 and the front shield member 123 are provided in order to suppress emission of an X-ray in a direction other than that which is necessary.
Here, when radiating the electron beam 201, a voltage is applied to the transmission-type target 124, and a high voltage is applied between the electron source 121 and the transmission-type target 124. Depending on the timing of application of the voltage to the transmission-type target 124 and the timing of radiation of the electron beam 201 from the electron source 121, there may be instances when the rear shield member 122 does not operate effectively, and thus an X-ray is emitted in an unnecessary direction.
The reason for this is that the voltage applied between the electron source 121 and the transmission-type target 124 increases as a slope relative to the application time. That is, even if voltage is already being applied to the transmission-type target 124, the transmission-type target 124 does not instantly reach a predetermined voltage. Therefore, immediately after starting voltage application, the voltage is low, so the electron beam is also radiated to an unnecessary area. For example, as shown in FIG. 8B, an electron beam is radiated also to the rear shield member 122, and thus an X-ray 205 is generated from the rear shield member 122. The X-ray 205 generated from the rear shield member 122 is unnecessary, and needs to be eliminated.
Even if the rear shield member 122 is provided as described above, depending on when the voltage is applied to the transmission-type target 124 and when an electron beam is radiated from the electron source 121, there is a possibility that an unnecessary X-ray will be generated.